As a support for an original film for a photomechanical process, an original film for a printed circuit board (PCB), etc. (hereinafter sometimes simply referred to as a "support"), a polyethylene terephthalate film having a relatively good dimensional stability against the variation of relative humidity (hereinafter sometimes abbreviated as "humidity") or temperature has been used. This is because an original film for a photomechanical process is liable to cause color drift in the lamination of four colors, i.e., cyan, magenta, yellow and black, when the positions of the exposure of the films are deviated from each other due to the variation of humidity or temperature. In the case of an original film for a printed circuit board, if image drift occurs due to the variation of humidity or temperature when printed circuit boards obtained by the exposure of these films are laminated on another, drilling for making through-hole may damage the circuit.
A polyethylene terephthalate film exhibits a relatively small dimensional change against the variation of humidity or temperature among high molecular films. Nevertheless, it is inevitably liable to cause some dimensional change. Thus, it is necessary that an original film for a photomechanical process, an original film for a printed circuit board, etc. be used in a special room that is considerably finely controlled to a humidity of about from 55 to 60% RH and a temperature of from 20.degree. to 27.degree. C. all the time and these films be exposed in this working atmosphere for a prolonged period of time, so that they are moisture- and temperature-conditioned. This drastically reduces the working efficiency.
JP-A-5-289236 (the term "JP-A" as used herein means an unexamined published Japanese patent application) proposes a photographic light-sensitive material comprising at least one hydrophilic image pre-colored layer on a support having a barrier layer made of SiO.sub.x (in which x is in the range of from 1.2 to 1.8) provided on a hydrophobic polyester film. In this photographic light-sensitive material, the support having an SiO.sub.x barrier layer exhibits a small dimensional change due to humidity as compared with the hydrophobic polyester film itself as a base material. In the case where such a photographic light-sensitive material is used in the field of PCB, however, the longer the working time is, or the larger the difference between the working atmosphere (particularly humidity) in which the photographic light-sensitive material is exposed to form an image and the subsequent working atmosphere is, the less sufficient is the dimensional stability against humidity thus obtained. The dimensional change shown in examples in JP-A-5-289236 is too large for practical use. This is mainly attributed to the fact that a barrier layer singly composed of SiO.sub.x (in which x is in the range of 1.2 to 1.8) cannot provide sufficient water vapor barrier properties.
Further, the support having SiO.sub.x as a barrier layer deposited on a hydrophobic polyester film exhibits a low transmission to the light of the near ultraviolet region because SiO.sub.x itself shows a considerably strong brown color. In particular, the smaller x is, the stronger is the brown color resulting in low transmission. Therefore, if such a support laminated with a light-sensitive resin such as resist is exposed to near ultraviolet rays (e.g., 350 nm) and developed in a desired pattern, it suffers from low transmission of light, requiring a higher output light source or a longer exposure time that reduces the productivity of printed circuit board.